Purpose of irqbalance

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Hi Neil,

smp_affinity for all the irqs are set to 0xf. However, i still see
that the irqs are routed only to cpu0. Is there any way of finding out
why is this the case? What could be the problem?

I think that if the affinity is set like 0x1, 0x2, 0x4... it will
work.. Wanted to find out the reason why cpu0 alone is processing all
the interrupts.

Also, from your explanation - irq balance uses only the smp_affinity
sysfs entry for the irq balancing. Is this correct?


On Wed, Sep 14, 2016 at 8:06 PM, Neil Horman <nhorman at tuxdriver.com> wrote:
> On Wed, Sep 14, 2016 at 06:52:23PM +0530, Sriram V wrote:
>> Hi,
>> Why is it that setting the smp_affinity alone is not sufficient for
>> irq to be routed to other cpus?
>> I tried doing that, but still the irq is only routed to cpu0.
>> In otherwords, why do we need a app to make the routing? how does
>> irqbalance app does that?
>> Regards,
>> Sriram
> Thats really two separate questions:
> 1) How does the irq smp_affinity setting affect irq routing
> 2) Why do we use irqbalance to handle affinity setting
> The answer to (1) is that the smp_affinity mask provides a user defined set of
> elligible cpus that can handle the corresponding irq.  For instance, setting irq
> 10's smp_affinity to 3 (0x11) means that cpu 0 or cpu1 is allowed to handle irq
> 10's interrupt.  The cpu which is actually selected is done by the hardware, and
> usually amounts to the lowest numbered cpu within the elligible set that isn't
> already handling an interrupt (which may explain, depending on how you set
> smp_affinity) why cpu0 is still handling the interrupt your interested in.
> The answer to (2) is convienience.  Manually setting smp affinity for all irqs
> is a tedious process as well as an ongoing one (consider hotplug or sriov use
> cases).  Irq balance can make adjustments to smp affinity to best balance irq
> workload according to different policies as irqs are allocated and freed.
> Hope that helps
> Neil
>> _______________________________________________
>> irqbalance mailing list
>> irqbalance at lists.infradead.org
>> http://lists.infradead.org/mailman/listinfo/irqbalance

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